The present invention relates to a transmitter and more particularly to a transmitter for a navigation system which transmits at least first and second signals and includes a phase regulator to control the phase of the first and second signals.
A transmitter of this kind is described in an article by H. Ruchardt entitled "Das Doppler-VOR-System", SEL-Nachrichten 16 (1968), No. 2, pp. 44 to 53. Section 2.1.2 describes the phase control between the output signals.
In some navigation facilities, different antennas radiate RF signals which are superimposed on each other in the radiation field. Only the signal obtained by the super-position can be used to determine the navigational information.
A Doppler-VOR or VOR ground station (VOR=VHF omnidirectional range), for example, emits carrier and sideband signals. The superposition of carrier and sideband signals in the radiation field gives an amplitude-modulated signal, with the amplitude of the resulting wave being dependent on the azimuth.
It is particularly important that a constant phase difference be set and maintained between the carrier and the sideband signals. Phase differences between different signals must also be held constant in other navigation systems, such as instrument-landing systems.
Phase differences are commonly regulated by means of phase-regulating circuits. Phase regulation per se is described, for example, in an article by K. D. Eckert entitled "Phasensynchronisation von HF-Oszillatoren gleicher und dicht benachbarter Frequenzen", Internationale Elektronische Rundschau 21 (1967), No. 6, pp. 153 to 157.
The conventional devices for regulating the phase difference between carrier and sideband signals permit precise regulation only if the sideband transmitter has a high stability itself. It then suffices to regulate the phase of the carrier relative to a reference signal. Since the high-frequency carrier wave of the sideband signals is reversed 180.degree. in phase after each half-cycle of the audio-frequency modulating signal, the sign of the phase difference cannot be determined with the conventional phase regulators. This, however, is necessary if the phase of the carrier wave of the sideband signal is to be maintained constant.